Modeling With Single Reference
First of all you have to guess how the back, bottom and part of the top look.

As far as proportions go, a box in orthographic projection at 20 degrees means that it projects about 93% of actual length of the front and 34% of the side. I asked a question here and got the answer for all the angles of a plane rotated in one axis. 



As a note the vertical stays at actual length and only the horizontal is shrinking. From the lines I measured, the side distance distance between the points is 2.15 times greater the the front one. However, it’s necessary to understand the form. For example, the back of the side is occluded, and the limit of the back is closer then that of the contour. 

If you are dealing with more complex angles, I gave an answer here about how to approach proportions. Hope this brief answer helps.